Photoresists are thin films of light sensitive materials which, when struck by a modulated light beam carrying information, change their solubility in certain solvents in the exposed region. The films can be developed with an appropriate solvent to transfer the information contained in the light beam as a surface relief pattern in the resist film. Recently, modulated electron beams have partially replaced light beams because they can be more highly focused to form higher density patterns, on the order of 1 micron or less. However, light sensitive resist materials are not necessarily sensitive to electron beams and most of them are highly complex organic materials. Thus a search for simple organic materials which are highly sensitive to electron beams and which can be developed to form high density relief patterns has been carried out by many workers.
Olefin-SO.sub.2 copolymers are known to be useful as positive electron beam resists. Investigations have been carried out by L. F. Thompson and M. J. Bowden, and reported in, inter alia, Journal Electrochem Society, Volume 20, No. 12, December 1973, pages 1722-26.
To prepare resist films of these copolymers, the copolymers are dissolved in a suitable solvent and solution cast, or, preferably, spun onto the desired substrate to form a uniform film about 0.1 to 0.5 micron thick. The solvent is removed, as by baking at 110.degree. C. for about 0.1-0.5 hours.
The copolymer films are then exposed to the beam of a scanning electron microscope, modulated to produce a relief pattern in the resist upon development with a suitable solvent. The polymer films are then post baked for 0.5 hour at 110.degree.C.
However, these copolymers cannot be employed when thicker resist films, on the order of 1 micron or more, are required, e.g., when the films are not to be developed through to the substrate and etched, because these films crack during solvent development. These cracks distort and even nullify the information in the developing relief pattern and have limited the use of olefin-SO.sub.2 copolymer resists to applications requiring only thin films. Further, the elevated temperatures, over 100.degree.C., employed to dry thin films cannot be employed when the substrate is sensitive to elevated temperatures. Also, prolonged heating, even at lower temperatures, e.g., about 50.degree.C., results in deterioration and warping of the copolymer films. Thus a method of preparing the films which will avoid the use of high temperatures and will prevent cracking of the films during development has been sought.